The direct and cross susceptibilities for the probe and conjugate fields as a function of the two-photon detuning δ/γ. The solid lines are the real parts, and the dashed lines are the imaginary parts. The excited state decay rate is γ = 2π × 5.75 MHz, and γ_c = 0.5γ. The hyperfine splitting of the ground state is ω₂₁ = 2π × 3.035 GHz. The detuning of pump 1 is Δ₁ = 174γ. The Rabi frequencies of Ω_P1 and Ω_P2 are Ω_P1 = 28γ and Ω_P2 = 30γ, respectively.